The present invention relates to optical switch arrays and, more particularly, to an optical switch array, of particularly compact geometry, in which arbitrary combinations of the inputs and outputs are explicitly addressable.
Integrated optical switches are well-known. For an early review of the art, see Lars Thylen, "Integrated optics in LiNbO.sub.3 : recent developments in devices for telecommunications", Journal of Lightwave Technology vol. 6 no. 6 (June 1988), pp. 847-861. Waveguides are created in a lithium niobate substrate by processing the substrate locally to increase the index of refraction. For example, the index of refraction of lithium niobate may be increased locally by diffusing titanium into the substrate. To divert light from one waveguide to another, the waveguides are coupled by local optoelectrical manipulation of their indices of refraction. Well-known examples of optoelectrical switches include directional couplers, BOA couplers, digital-optical-switches and x-switches. Depending on the voltage applied to such a switch, light is thus partly or completely diverted from an input waveguide to an output waveguide.
By appropriately combining waveguides and switches, a switch array is formed to switch light from a plurality of input waveguides among a plurality of output waveguides. A variety of switch array geometries are known. FIG. 1A is a conceptual illustration of a switch array of one such geometry: crossbar geometry. A set of input waveguides 10 crosses a set of output waveguides 12. At the crossing points, the waveguides are coupled by 2.times.2switches 14. For simplicity, only four input waveguides 10 and four output waveguides 12 are shown in FIG. 1A. Typically the numbers of input waveguides 10 and output waveguides 12 are equal powers of 2, up to a practical maximum of 32.
FIG. 1B shows, schematically, the actual layout of the switch array of FIG. 1A. Switches 14 are shown as directional couplers, in which parallel segments of the waveguides are flanked by electrodes (not shown) to which the coupling voltages are applied. Note that input waveguide 10a leads directly into output waveguide 12a, that input waveguide 10b leads directly into output waveguide 12b, that input waveguide 10c leads directly into output waveguide 12c, and that input waveguide 10d leads directly into output waveguide 12d. To allow arbitrary coupling of inputs to outputs, three auxiliary waveguides 11a, 11b and 11c are provided. Waveguides 10a-12a and 10b-12b are coupled in switch 14a. Waveguides 10b-12b and 10c-12c are coupled in switches 14b and 14c. Waveguides 10c-12c and 10d-12d are coupled in switches 14d, 14e and 14f. Waveguides 10d-12d and 11a are coupled in switches 14g, 14h, 14i and 14j. Waveguides 11a and 11b are coupled in switches 14k, 14l and 14m. Waveguides 11b and 11c are coupled in switches 14n and 14o. Note that switches 14g, 14k and 14n actually are 1.times.2 switches, that switches 14j, 14m and 14o actually are 2.times.1 switches, and that there is no switch corresponding to the lowermost 2.times.2 switch 14 of FIG. 1A. (A 1.times.2 switch is a 2.times.2 switch with one input deactivated; a 2.times.1 switch is a 2.times.2 switch with one output deactivated.)
Switch arrays based on geometries such as the crossbar geometry of FIGS. 1A and 1B can be used to divert input signals to output channels arbitrarily. Signals from any input channels can be directed to any output channel, and even to multiple output channels, in broadcast and multicast transmission modes.
Despite the conceptual simplicity of the crossbar geometry of FIGS. 1A and 1B, this geometry has been found inferior, in practice, to two other geometries, the tree geometry, illustrated in FIG. 2, and the double crossbar geometry, illustrated in FIG. 3. FIG. 2 shows the tree geometry, for four input waveguides 20 and four output waveguides 22. Waveguides 20 lead into a binary tree of 1.times.2 switches 24. Waveguides 22 emerge from a complementary binary tree of 2.times.1 switches 26. The highest order branches of the binary trees are connected by intermediate waveguides 28. FIG. 3 shows the double crossbar geometry, for four input waveguides 30 and four output waveguides 32. Each input waveguide 30 traverses four 1.times.2 switches 34a, 34b, 34c and 34d. Each output waveguide 32 traverses four 2.times.1 switches 36a, 36b, 36c and 36d. The remaining outputs of switches 34 are connected to respective inputs of switches 36 by intermediate waveguides 38. Note that, in principle, switches 34d and 36a are not needed, because input waveguides 30 could lead directly to switches 36d and output waveguides 32 could emerge directly from switches 36a; but, in practice, the illustrated configuration has been found to reduce cross-talk.
The tree and double crossbar geometries require larger numbers of switches than the equivalent crossbar geometry. Nevertheless, the tree and double crossbar geometries have certain advantages over the crossbar geometry:
1. The tree and double crossbar geometries have lower worst-case crosstalk than the crossbar geometry.
2. In general, the path from a particular input waveguide to a particular output waveguide through a crossbar switch array is not unique. Therefore, computational resources must be devoted to reconfiguring a crossbar switch array in real time. In a tree switch array or in a double crossbar switch array, the path from any particular input waveguide to any particular output waveguide is unique, so it is trivial to compute how to reconfigure such a switch array in real time.
3. To prevent loss of optical power by radiation, the intermediate waveguides of an optical switch array must have gentle curvature. In the case of the crossbar geometry, this requires that the switches be arranged in a diamond pattern, as illustrated in FIGS. 1A and 1B. This is a less efficient packing of the switches than, for example, the rectangular matrix pattern of the double crossbar switch as illustrated in FIG. 3.